1. Field of the Invention
The present invention relates to a pneumatic tire in which a protruding stripe is provided in a groove bottom of a groove portion of a tread surface, and a tire mold for molding the pneumatic tire.
2. Description of the Related Art
A tread surface of the pneumatic tire is generally provided with a groove portion such as a circumferential groove, a lateral groove or the like, and a land portion divided by the groove portion, and various tread patterns are formed in correspondence to a required tire performance and use condition. There is a case that a stone on the road is pinched in the groove portion at a time of traveling on a gravel road or the like which is not leveled. If the tire rolls under such the state, a force which tends to expand the groove portion is repeatedly applied, or the pinched stone makes an intrusion into the groove bottom, whereby there is a problem that a damage such as a groove bottom crack or the like is caused.
With regard to this problem, there has been proposed a structure in which a protruding stripe as shown in FIG. 17 is provided in a groove bottom of a groove portion, thereby preventing a stone biting, as described in Japanese Unexamined Patent Publication No. H10-35224, No. 2000-185525, and No. 2002-29218. However, if a small stone S is pinched between a protruding stripe 80 and a groove wall 9b as illustrated, there is a case that a side surface of the protruding stripe 80 is locally deformed so as to hold the small stone S in a wrapping manner. Therefore, it is not said that a stone biting resistance is sufficient.
Further, since a volumetric capacity in a side of a groove bottom 9a becomes smaller by setting the protruding stripe 80, a drainage performance is lowered in a state in which a wear has advanced, and a wet performance (a wet road traveling performance) may be deteriorated. On the other hand, if an angle θ3 of the groove wall is made small so as to secure a volumetric capacity in a side of the groove bottom 9a, there is generated a problem that the stone biting tends to be generated. Further, there is a concern of groove bottom crack starting from a root of the protruding stripe 80.
Conventionally, there is a tendency that a rigidity of the protruding stripe is increased for increasing a resistance against the stone intrusion, and Japanese Unexamined Patent Publication No. 2002-29218 describes a protruding stripe formed in a trapezoidal cross sectional shape obtained by gradually decreasing a width from a groove bottom toward an upper surface. However, in the case that the small stone is bitten between the protruding stripe and the groove wall as mentioned above, it is known that the small stone is strongly held due to its high rigidity, thereby making it hard to discharge out of the groove.
Further, Japanese Unexamined Patent Publication No. H10-226206 describes a pneumatic tire provided with a protruding stripe in a groove bottom of a groove portion, however, since such the protruding stripe is a small protruding stripe such a degree that a height thereof is equal to or less than 1 mm, and a plurality of protruding stripes can be provided in parallel in the groove bottom, the protruding stripe can not prevent the stone biting as mentioned above.